evaluation of gravelless pipe systems for onsite wastewater

EVALUATION OF GRAVELLESS PIPE SYSTEMS FOR ONSITE
WASTEWATER TREATMENT ACROSS MINNESOTA
INTRODUCTION
S. H. Christopherson, D. M. Gustafson, J. L. Anderson
Approximately 30% of Minnesota's residents rely on onsite technologies for their wastewater
treatment. Gravelless pipe systems became a standard choice for trench systems in 1989 in
Minnesota Rules Chapter 7080. This system is called gravelless because no rock or gravel is
placed in the trenches. The gravelless pipe evaluated in this study was 10” inner diameter
corrugated polyethylene tubing covered with a permeable nylon fabric. Two rows of holes,
approximately 1/2 inch in diameter are located at the four o’clock and eight o’clock position. Sizing
requirements for these systems are based on research performed at the University of Minnesota in
the early 1980s (Anderson, 1985).
Figure 1. Installation of gravelless pipe
The Minnesota Onsite
Sewage Treatment
Contractors Association
(MOSTCA) received
concerns from its members
about gravelless pipe
systems in 1999. MOSTCA
contacted several suppliers
of gravelless pipe who
agreed to fund a research
study to see if problems do,
in fact, exist. The purpose of
this study was to determine
the reason(s) for success
and failure of Individual
Sewage Treatment Systems
that use gravelless
technology in Minnesota.

PROCEDURES
Of the fourteen systems evaluated, eleven systems had experienced problems (surfacing to the
ground and backups into home) while the remaining three did not exhibit any obvious signs of
failure. Typically, if the gravelless system exhibited functional problems, either the homeowner or a
septic system professional contacted the University for assistance to determine why there was a
problem. A system usage survey was given to each homeowner. Information requested included:
flow data (measured or estimated), number of people living in home, design (including size and
location of septic tank and soil treatment system), age and performance of system, maintenance
history and usage of garbage disposal, dishwasher, laundry, water softener, antibacterial soap and
cleaning supplies. Individual locations had a detailed site analysis performed to evaluate the soil,
system, location and condition.
The trenches were evaluated to determine the depth of ponding in each trench and amount of
biomat development. Soil borings were conducted to determine depth to water table or other
limiting soil conditions. Soil samples were taken to determine soil texture classification from particle
size analysis distribution analysis, (US Department of Agriculture, 1993).
Effluent samples were collected from the septic tank and from inside the gravelless pipe. These
were taken as grab samples. Samples were taken at the 40% liquid depth at the outlet of the
septic tank and in the gravelless pipe with a Masterflex pump. Samples were analyzed for
dissolved oxygen (DO), biological oxygen demand (BOD), and total suspended solids (TSS). All
samples were collected and analyzed in accordance with Standard Methods for Examination of
Water and Wastewater (American Public Health Association, 1992).

RESULTS AND DISCUSSION
System locations are shown in Figure 2. The number in the county represents the number of
systems evaluated. The map illustrates that the study was concentrated in and around the
northern metropolitan area and central Minnesota. These locations were selected based on
homeowner and/or contractor requests. As a result, this provided a focus on problem areas and, in
particular, sites with fine sand soil textures.
Figure 2. Location of Gravelless Pipe Systems Evaluated.
Soil texture was identified by the feel method at each site and followed up with a sieve analysis to
confirm those field results. Table 1 summarizes the soil textures and number of systems evaluated
with and without problems for each soil texture. Sieve results are included in the individual reports.

Soil
Classification
Results of Effluent Analyses
Table 1. Soil Classification by System
# of systems
with problems
# of systems with
no problems
Sand 3 1
Fine sand 5 2
Loam 1 >NA
Sandy clay
loam
1 NA
Loamy sand 1 NA
The results from the septic tanks and gravelless pipe for BOD, TSS and DO were all within normal
levels to be expected in a septic tank effluent. The BOD values were all below 220 mg/1 and the
TSS values were below 65 mg/1. Many of these were skewed because the first thing many
homeowners do when there is a problem is to pump the septic tank. This pumping affects these
results and the typical operation of these tanks may have been different. Visual inspection
indicates that there is reason for concern that some of these tanks may not be watertight. This was
not apparent during the site evaluations but these were not done during the spring when such high
water table conditions may exist, in the vicinity of the tanks.
Flow Results
Five of the eleven homeowners installed flow meters to monitor the flow. Flows were monitored for
periods of seven to fourteen days. Three of the homes had flows well below the design levels.
Two were close to or above their design flow rates. It is not possible to make a definitive
conclusion based on such limited time periods. However, consistent flows at or above design
values will shorten the life and reduce performance in any onsite system.
The other homes refused or could not install a flow meter. In these instances we do not know if the
system is being hydraulically overloaded. Based on the number of people living in these homes
and the number of water using devices, none of them appear to be exceeding the daily estimated
sewage flow rates.

Homeowner Usage Results
The table below summarizes some of the homeowner water use patterns.
House Age Garage
Disposal
Table 2. Homeowner Data Summary
Water
Softener
Dish
washer
Anti-bacterial
Soap
Long Term
Prescription
Drug(s)
Date of Last
Pumping
1 10 x x x x Summer
1999
2* 2 Spring 1999
3 4 na na na na na na
4 9 x x Summer
2000
5 4 x x x x Winter 1999
6 5 x x Summer
2000
7 7 x x x Fall 98
8 6 na na na na na
9 5 x x Summer
2000
10 5 x x Summer
2000
11 10 na na na na na na
12 5 x x Summer
2000
13* 3 x x x Spring 2000
14* 7 x x x Fall 1999
* Systems with no apparent problems
Reasons for Failure
The average age of the gravelless system evaluated was six years with a minimum age of two and
the maximum of ten years. Age is important because Chapter 7080 became mandatory in 1996.
Previously using 7080 was voluntary, so a person could choose to downsize gravelless systems by
20-50%, which was a manufacturers recommendation.
This evaluation looked at systems using various ‘brands’ of gravelless pipe. There are four main

suppliers of gravelless pipe in Minnesota. Problems were seen among all four brands. The
products, particularly the fabric, vary by brand and year of installation.
Hydraulic failure of an onsite system is often caused by either daily sewage flows exceeding the
design rate, or an incorrect evaluation of the long-term acceptance rate. The long-term
acceptance rate can be negatively affected by improper site ands soils evaluation, improper system
location, improper construction practices and lack of maintenance (Anderson, 1982). One or more
of these factors can result in system failure.
All the systems evaluated with problems were either designed or installed improperly. Not one site
with problems was designed according to Chapter 7080. At each site, all trenches were ponded
with effluent. Four primary factors were identified that negatively affected the systems.
1.
2.
3.
Nine of the eleven problem systems evaluated were undersized according to design criteria.
Of these nine, two were designed 50% too small, four - 40% too small, one - 30% too small
and two - 20% too small. The table below from Chapter 7080.0170 indicates the required
sizing factors. Two of these systems did not have three feet of separation as well. A
common error was to size a fine sand as a coarse or medium sand or as a sandy loam. This
general reduced size, combined with near design flows will limit the long-term performance of
these systems.For example, a system for a 3 bedroom is designed to treat 450 gallons per
day. If a system is designed based on a medium sand texture it may, in fact, be large enough
for 225 gallons per day flow if the soil texture is fine sand.
Figure 3. Soil Characteristics and Sizing Factors According to Chapter 7080
Three of the eleven were designed and installed with less than the required three-foot
separation to the water table. Chapter 7080 requires three feet of unsaturated soil be
present beneath the trench to adequately treat sewage. (See Chapter 7080.0060). At one site
the standing water level was visible at the same depth as the gravelless pipe. At the other
two, redoximorphic features (mottling) were used to determine the seasonally high water
table. Two of these systems were undersized as well. One was 30% too small and the other
was 40% too small.
One of the systems was installed improperly. Trench systems must be installed level
following land contours. (See Chapter 7080.0060). There was a four-inch drop from one end
of the pipe to the other over the fifty-foot trench length. Therefore as the sewage seeks its
own level it will surface at the point of lowest elevation. This occurred at the far end, where
the soil cover was lower than the elevation of the outlet from the distribution box.

4.
All systems evaluated had biomat development, varying from one to four inches surrounding
the pipe. When septic tank effluent is applied to soil, a biomat forms at the interface of the
soil and the gravelless pipe. Development of the biomat occurs more quickly under anaerobic
conditions. Therefore, separation distance to the water table is critical to provide an aerated
soil below the biomat.
Table 3. System Summary
House Age Amount of Biomat
(inches)
Problem(s)
1 10 3 Does not have 3 feet of separation
40% too small
2* 2

The biomat extends into the soil; therefore
the fabric is not inhibiting biomat development
in the soil (Anderson, 1982). When the
biomat was removed from around the pipe
the effluent would run out of the pipe. This is
a normal function, it is why effluent ponds in
any sewage treatment trench. This indicates
that the effluent was being prevented from
leaving the gravelless pipe by the biomat.
There is the possibility that the pipe could be
plugged from the inside as well, but this was
not apparent at any of the sites evaluated in
this study. There was no apparent
accumulation of sludge or other material in
the pipe.
All three systems that did not have three feet of separation (two of which were downsized as
well) had 3 inches of biomat surrounding the pipe. All of systems that were sized 30-50% too small
had 2-4 inches of biomat surround the pipe. These values indicate the systems are probably
hydraulically overloaded.
Reasons for Success
1. Two of three systems visited had two septic tanks. This is above what Chapter 7080
requires. It provides additional settling time and storage space.
2. All three systems were sized appropriately based on estimated daily sewage flow and soil.
3. All three systems had three feet or more of vertical separation to the water table.
4. On average, only half of the trenches had effluent ponded in the pipe.
CONCLUSIONS
1. The sample size was too small to provide definitive answers about whether gravelless pipe
has problems.
2. All of the systems in this study evaluated had design or construction errors. These errors
strongly contributed to the failure of the systems evaluated. These errors may be due to lack
of knowledge by designer/installer and local unit of government staff, pricing/bidding issues or
simply a mistake.
3.
The sites we evaluated appeared to be plugging from the outside of the pipe due to biomat
development that would indicate that the problem is not due to the product itself, but how it is
sized and installed.

RECOMMENDATIONS
1. More systems should be evaluated to determine if there is a problem with gravelless pipe.
2. If there is doubt about how a soil should be sized the larger soil-sizing factor should be
chosen.
3. Sieve analysis should be done with sandy soils that appear to have large amounts of fines.
4. A technique to accurately judge the amount of buildup on the fabric material should be
developed. Tests that have been done are unable to differentiate between plugging on the
inside or outside and soil particles that remain attached to the media.
ACKNOWLEDGEMENTS
The Minnesota Onsite Sewage Treatment Contractors Association (MOSTCA) funded this
project. Thanks to the homeowners, contractors and local units of government that
assisted.
For more information about the individual sites evaluated please call (612) 625-7243 or email
heger001@umn.edu
REFERENCES
American Public Health Association. 1992. Standard Methods for Examination of Water and
Wastewater (18 th 1.
edition). American Public Health Association, Washington D.C.
2.
3.
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5.
Anderson, J. L., R. E. Machmeier and M. P. Gaffron. 1985. Evaluation and Performance of
Nylon Wrapped Corrugated Tubing in Minnesota. Proceedings of the Forth National
Symposium on Individual and Small Community Sewage Systems. American Society of
Agricultural Engineers, St. Joseph, MI.
Anderson, J. L., R. E. Machmeier and M. J. Hansel. 1982. Long-Term Acceptance Rates of
Soils For Wastewater. Proceedings of the Third National Symposium on Individual and Small
Community Sewage Systems. American Society of Agricultural Engineers, St. Joseph, MI.
Minnesota Pollution Control Agency. 1999a. Minnesota rules chapter 7080 – individual
sewage treatment systems program. Office of Reviser of Statutes, St. Paul, MN.
United States Department of Agriculture. 1993. Soil Survey Manual. US Government
Printing Office, Washington, DC.